Cellulose is a substance of great industrial importance having numerous applications. Primary source of cellulose in industrial applications is wood-based cellulose pulp. However, in using wood-based raw-material there are several problems such as environmental issues relating to unsustainable use of land and soil and heavy energy consumption required to grow, harvest and process wood-based material. These issues have created a need to find, on one hand, alternative sources of cellulose for producing new cellulosic materials. Further, the industry is constantly searching for more economical methods and raw materials to produce high quality cellulosic products.
In nature, native cellulose is always in a microfibrillar form, being part of wall structures of the plant cell. In primary cell walls, especially in parenchyma cells, cellulose microfibrils are distributed randomly forming a flexible membrane layer together with other polysaccharides, such as pectin and hemicelluloses. In certain plant species, an additional secondary wall structure is formed after the cell is fully-grown, especially in various wood species. In the secondary cell walls, the microfibrils are highly aligned mostly in the same direction and tightly bound to each other through hydrogen-bonding and covalent lignin bridges, forming a very rigid structure.
Cellulose microfibrils located either in primary or secondary walls are structurally very similar, if not identical (Thomas et al., Structure of Cellulose Microfibrils in Primary Cell Walls). Both type of microfibrils consist of highly aligned cellulose macromolecule chains forming mechanically strong cellulose crystals with hydrogen bonded parallel polymer chains. The microfibrils are generally considered to contain only few faults along their axis, although the degree of crystallinity varies between plant species being generally higher for microfibrils obtained from secondary walls. It is commonly understood that, depending on the plant specie, 18, 24, or 36 cellulose chains form the smallest aligned structure, which is known as cellulose elementary fibril having diameter of a few nanometers and lengths up to tens of micrometers. These nanofibers have remarkable mechanical properties: tensile strength in the order of 138 GPa and toughness in the order of 13 GPa. Thus, cellulosic microfibrils are of interest if they can be dissociated into single fibres.
Although the secondary cell walls, for example in wood, are rich of cellulose microfibrils, isolation of the structures without damaging the fibrils itself is very difficult. Also, the needed fibrillation process is complicated, expensive, and often a chemical pre-treatment is needed prior to fibrillation. Plant tissues made of primary cell walls, however, form an alternative source for the separation of the microfibrils. Cells with primary walls are common for example in all fruit and vegetable species. These plants are mainly composed of parenchyma cells, i.e. ground tissue that generally constitutes the “filler” tissue in soft parts of plants. They have thin but flexible primary cell walls and the secondary cell wall is usually absent. The parenchyma tissue has a variety of functions, for example, to store starch in tubers, such as potato and cassava or storage of sucrose in sugar beet and sugar cane pith. In addition to the loose deposition of the microfibrils in the primary walls, the other polysaccharides present in the cell wall have a more charged nature, which allows for more facile processing to separate the individual microfibrils.
In certain plant species, primary and secondary cell wall structures co-exist. For example, in various grasses the plant structure is composed of rigid outer shell made of macroscopic cellulose fibers with a thick secondary cell wall and a soft internal core, a pith tissue, made of predominantly parenchyma cells with a thin primary wall. The core part often contains also a small fraction of cellulose fibers. As was described, the cellulose microfibrils in primary walls are easier to separate than the fibrils in secondary wall structures.
Sugar cane is an economically important plant with an estimated worldwide harvest of 1.83 billion tonnes. Sugar cane is 3 to 6 metres tall and consists of stout jointed stalks, rich in sucrose. Mature stalks consist of 11-16% fibre, 12-16% soluble sugars and 63-73% water. The stalks themselves consist of a hard outer shell called dermal tissue which functions to water proof the inner core and to strengthen the stalk, allowing it to grow tall. The soft inner core consists of the ground tissue that has filled around the vasculine tissue. More specifically, the ground vasculine tissue consists mainly of parenchyma cells.
Sugar canes are typically processed by mechanically crushing the stalks to remove the sugar rich juice. Thereafter, the spent fibrous matter, called bagasse, can be used in the production of biofuel or to manufacture pulp for paper and board products or building materials. Especially for the paper and pulp industries the bagasse is stored wet in order to assist in the removal of the short pith fibres and the soft parenchymal cells, i.e. the softer inner core, which impede the paper and board making process. Various mechanical processes have been developed to assist depithing, including hammer milling and dry fractioning. The resulting fraction is called bagasse pith or spent bagasse pith.
Spent bagasse pith comprises predominantly pectin, arabinogalactan and cellulose. Other naturally occurring biological constituents of bagasse pith, such as fats proteins, soluble oligosaccharides, and other low molecular weight components, are largely extracted from sugar cane during the removal of sucrose therefrom. The remaining polysaccharides in bagasse pith generally conjugated, particulate cell residuals having morphologies generally characteristic of parenchymal cells found in certain higher plants. Often, the bagasse pith also contains a small number of cellulose fibres even after depithing process: although the pith tissue is predominantly composed of parenchyma cells a small number of cellulose fibers are present to stabilize the soft tissue in the original plant structure. Few economical uses have been found for bagasse pith. For example bagasse pith is a material that spoils rapidly and consequently constitutes a local environmental problem. Thus, alternative uses for these waste streams are needed.
The following patents represent the current state of the art of processing bagasse. Thus far bagasse based products have been manufactured by fibrillating the cellulosic component from bagasse extracted from the secondary wall structures and/or the extracted using ionic liquids.
CN 103422379 discloses a method for preparation of dried cellulose fibres from bagasse. The method comprises treating by acid and base, followed by mechanical grinding to obtain uniform biomass.
CN 102505546 discloses a method for preparation of homogeneous cellulose nanofibres from sugar cane, wherein the cellulose fibres are extracted in ionic liquids followed by high-pressure fibrillation. The resulting product is mainly based on secondary wall structures, not parenchymal cellulose.
US 20080227753 A1 discloses a method for preparation of sugar cane bagasse fibers by pulverising a frozen and dried bagasse for use as a dietary supplement.
Even though some uses for cellulose rich food/feed waste streams are envisaged, due to logistical requirements it would be beneficial to be able to process the raw material locally without need for transportation.
Further, it would be advantageous to be able to process high volume raw materials into high quality products.
It is an aim to solve or alleviate at least some of the problems related to prior cellulosic materials and their production methods, as discussed above. In particular, an aim is to provide from a novel raw material source for manufacturing cellulosic materials that have good rheological and/or binder properties in aqueous suspensions and good mechanical and/or binder properties in a dry state.
Another aim is to provide a new use for the spent bagasse pith.
Another aim is to provide additive compositions for industrial applications, paper and board manufacture in particular.
Another aim is to provide new high performance articles of manufacture.